The present invention relates to power supplies, and more particularly to the AC power adapters for power supplies.
Alternating current (AC) power adapters are well known in the art. FIG. 1 illustrates a conventional circuit for an AC power adapter. The adapter circuit 100 comprises an electro-magnetic compatibility (EMC) filter 102 which can be coupled to an AC power source 104. The EMC filter 102 allows the adapter circuit 100 to meet the EMC requirement. The EMC requirement is well known in the art and will not be further described here. The EMC filter 102 is coupled to the input nodes of a rectifier bridge 106, as illustrated, for rectifying the AC voltage from the AC power source 104. Coupled to the rectifier bridge 106 is a conversion circuit 108 for converting the rectified AC voltage from the rectifier bridge 106 to a direct current (DC) voltage for the system 110.
However, when the system 110 is disconnected, or the “start” key on the keyboard is in the “stop” state, the circuit 100 continues to draw power from the power supply 104. The adapter circuit 100 draws a standby power whether the system 110 is connected to the adapter or not. Typically, the switching losses while the AC adapter is in idle mode reach as high as 5W and causes the plastic housing of the adapter (not shown) to heat, resulting in wasted power.
Accordingly, there exists a need for a circuit for an AC adapter which reduces the amount of power drawn from an AC power source when a system is disconnected from the adapter. The present invention addresses such a need.